Hygiene products such as disposable diapers and sanitary napkins are generally composed of a topsheet, a backsheet, a hot-melt adhesive, a stretchy material, a water-absorbent resin, a pulp fiber, and like hygiene materials. In such hygiene materials, many synthetic resins are used, and thus the hygiene materials may have an odor originating from raw-material components. Since these hygiene products are worn on a human body, even a slight odor results in user discomfort, and therefore development of an odor-free material is desired.
Among the above hygiene materials, hot-melt adhesives especially have a strong odor originating from the raw materials used. Since the odor tends to spread when the hygiene products are used, there is demand for reducing the odor.
Hot-melt adhesives contain about 30 to 50% tackifier to improve adhesive strength. Derivatives of rosin, which is a natural resin, are widely used as the tackifier.
Common rosin-based compounds are hard and brittle at room temperature, and these properties causes, for example, the following problems especially when such a rosin-based compound is incorporated into an acrylic adhesive: they lead to a decrease in tack; the adhesive hardens at low temperatures, resulting in unsatisfactory adhesive strength; and a phenomenon such as zipping is caused. In addition, rosin-based compounds are structurally susceptible to oxidative degradation due to heat or light, leading to deterioration of the adhesives.
Adhesive compositions prepared using a liquid rosin-based tackifier resin as a tackifier have been proposed (see Patent Literature 1).
Patent Literature 1 does not clearly disclose the structure and glass transition temperature (Tg) of the conventional liquid rosin-based tackifier resin used, and the adhesive showed insufficient adhesion performance at low temperatures. Moreover, in Patent Literature 1, no measure is taken for the stability of the adhesive to heat or light.
Further, an adhesive containing a rosin-based compound as a tackifier and having an odor level low enough for the adhesive to be used as a hygiene material has not yet been developed.